1. Field of the Invention
This invention relates generally to the polishing of glasses, semiconductors, and integrated circuits. More particularly, this invention relates to the surface preparation of articles wherein a more rapid rate of polishing is desired.
2. Description of the Prior Art
Polishing solutions, or slurries generally consist of a solution which contains a concentration of abrasive particles. The part, or substrate, is bathed or rinsed in the slurry in conjunction with an elastomeric pad which is pressed against the substrate and rotated such that the slurry particles are pressed against the substrate under load. The lateral motion of the pad causes the slurry particles to move across the substrate surface, resulting in wear, volumetric removal of the substrate surface.
The rate of surface removal (polishing rate) is largely determined by the magnitude of the applied pressure, the velocity of pad rotation and the chemical activity of the abrasive particle. While virtually any particle of sufficiently small size may be used for polishing, economically useful high polishing rates are exhibited by a relatively small number of compounds. For most substrates (e.g., SiO.sub.2 or silicates) the highest rates are found for formulations composed primarily of CeO.sub.2 or ZrO.sub.2. In consequence, there is a large body of prior an describing the composition and preparation of polishing slurries based on these two oxides.
Extensive efforts have been made to develop additives which accelerate the rate of polishing in order to make the polishing process more economical. Such accelerants may be generally classified as etchants, which would by themselves dissolve the substrate, or polishing compound accelerants, which increase rates when added to the abrasive itself. Etchant accelerants, such as described in U.S. Pat. No. 4,169,337 are commonly employed in conjunction with SiO.sub.2 abrasives to polish silicon wafers. These additives can be classified into two categories; (1) additives that increase or buffer the solution pH (e.g. organic amines), or (2) organic compounds, generally amines, that may additionally increase the Si corrosion rate by complexing or sequestering Si (e.g., ethylene diamine or piperazine). These classes of etchant accelerants are distinctly different than the accelerants employed in the present invention.
A variety of polishing compound accelerants have been described. They can be classified into two main categories; (1) Additives which are coprecipitated with the base abrasive prior to calcination, and (2) water soluble additives to the final polishing slurry. Examples of prior art belonging to the first category are found in U.S. Pat. No. 3,262,766 (Nonamaker), U.S. Pat. No. 3,768,989 (Goetzinger and Silvernail) and U.S. Pat. No. 3,301,646 (Smoot). These examples are important, as they illustrate the primary prior art pathways for coprecipitating activating substances.
Nonamaker teaches the incorporation of small amounts of SiO.sub.2 (&lt;5%) to a mixture consisting primarily of rare earth oxides (including CeO.sub.2) prior to calcination in order to accelerate polishing rates in the final calcined product. The precise mechanism of this effect is not understood.
In a similar fashion, Goetzinger and Silvernail taught coprecipitation of rare earth carbonates, primarily cerium carbonate, together with Wollastonite (calcium metasilicate). The co-precipitate was subsequently calcined to yield an activated final product. Once again, the precise mechanism of the activation was not disclosed.
Smoot taught the deliberate incorporation of Calcium or other divalent ions (e.g., Mg.sup.2+) into zirconium oxide to produce calcium-stabilized cubic zirconium oxide, a material which is widely used as a structural ceramic. The process consisted of dry batch mixing of ZrO.sub.2 and the stabilizing compound, typically CaCO.sub.3, followed by calcination of the mixture at elevated temperature (.sup..about. 2100.degree. F.) to form a cubic ZrO.sub.2 product. The stabilized cubic zirconia was found to have an accelerated polishing rate relative to the normal monoclinic phase of zirconia obtained without addition of the calcium accelerant.
The second pathway for activation is the activation of slurries by addition of water soluble additives to the final solution: As reviewed by Silvernail ("Mechanism of Glass Polishing", Glass Industry, vol. 52, pp. 172-5, 1971), addition of Ce(OH).sub.4 to polishing slurries can produce significantly increased polishing rates. In particular, some previously inactive oxides, such as Tb.sub.4 O.sub.7 showed high polishing rates after Ce(OH).sub.4 addition. Other compounds have also been used as accelerants. Shlishevskii and Migus'kina (Sov. J. Opt. Technol., vol. 44, pp. 680-1, 1977) demonstrated as much as 2.times. improvement in polishing rate when 2% ammonium molybdate, 1% Mohr's salt (NH.sub.4)SO.sub.4.FeSO.sub.4, or 1% zinc sulphate was added to a CeO.sub.2 -based polishing slurry. The basis of the effect was ascribed to complexation of silicate reaction products with the additive compounds so as to prevent their redeposition back onto the substrate surface.
The prior art methods for enhancing polishing activity suffer from a number of deficiencies. First, while etchant additives may increase the overall rate of surface removal of the substrate, their action is isotropic, i.e., they attack all portions of the exposed substrate surface regardless of position. This leads to significantly degraded surface roughness and texture in the polished substrate. Their incorporation is therefore generally considered to be undesirable for slurries used to prepare high quality surfaces (i.e. final polishing). This is particularly true for the polishing of Si wafers.
As regards polishing compound additives, the principal deficiency of adding additives prior to calcination is that rates cannot be adjusted subsequent to formation of the final polishing compound. An additional deficiency is that the technique cannot readily be applied to some polishing abrasives of technical importance, particularly SiO.sub.2, which is commonly used for Si wafer and integrated circuit polishing. Solution additives, such as Ce(OH).sub.4, have not given consistent activation and cannot be used with SiO.sub.2 -based polishing slurries due to gelation.
Yet another obvious way of increasing the polishing rate of a slurry with low rate (e.g. SiO.sub.2) would be to simply add to it a portion of another slurry (e.g., CeO.sub.2). While this has not been the subject of prior art disclosures, it is a common practice in the polishing art. This technique suffers from two deficiencies. First, the rate of increase is linearly proportional to the amount of the second slurry added. Thus, to achieve a substantial amount of acceleration, a significant fraction of the second material must be added. Second and more critical, addition of the second slurry changes the particle size distribution of the original slurry unless the two particle size distributions are precisely matched. While this may be possible, it is generally not economically feasible. This is particularly true in the case of colloidal silica slurries such as are used in Si wafer polishing. These slurries have extremely small particle sizes, typically 50-100 nm. In contrast, all known commercial CeO.sub.2 -based slurries have mean particle diameters in excess of 1000 nm. Incorporation of such larger particles would have a catastrophic effect on the quality of the Si wafer surfaces produced after polishing.
From the above, it is clear that an additive which could increase polishing rams without increasing the static corrosion of the substrate, which could be applied to a variety of abrasive types, particularly SiO.sub.2, subsequent to particle formation, and which could be applied without alteration of the original slurry particle size would be a significant improvement over prior art.
Accordingly, it is the object of this invention to provide an improved means of increasing the polishing rate of slurries without increasing the overall corrosiveness of the polishing solution, which can be easily applied to a variety of abrasive particles, particularly SiO.sub.2, and which can be employed in a manner which does not alter the original particle size.
It is also an object of this invention to provide polishing slurries with significantly improved performance which are prepared by said means.
These and other objects of the invention will become apparent to those skilled in the art after referring to the following description and examples.